1. Field of the Invention
The present invention relates to a signal quality detector circuit used, for example, in a digital microwave radio system. Signals transmitted over microwave radio lines (simply called lines hereafter) are subject to radio interferences such as fading and distortion while being transmitted through the air. Such interference causes a deterioration in transmission quality. Generally, digital microwave radio systems have a plurality of regular lines and at least one alternative line. When deterioration in transmission quality is detected on a regular line that is currently being used, the regular line is replaced with an alternative line. Therefore, it is necessary to detect both signal quality and deterioration in transmission quality correctly and quickly.
2. Description of the Related Art
FIG. 1 is a schematic block diagram of a system for practicing an existing method of detecting signal quality in a digital microwave radio system. At the sending end, a digital data signal is input to transmitter 81. A parity generator (called PG hereafter) 82 adds a parity bit to, for example, every 7 data bits. Transmitter 81 performs a digital-to-analog conversion to convert the digital data and parity bit to an analog signal. The transmitter 81 then modulates the converted baseband signal using, for example, quadrature amplitude modification (called QAM or QAM modification hereafter), and transmits the modulated signal over a regular line 100a.
At the receiving end, the signal received via regular line 100a is input to receiver 91. The receiver 91 demodulates the QAM-modulated signal and converts the demodulated analog baseband signal into a digital data signal. A parity checker (called PC hereafter) 92 checks the baseband bit signal for proper parity. When the number of parity errors in a unit time (or error rate) exceeds a predetermined number (or value), PC 92 sends a command to the sending end over control line 300a to switch regular line 100a to an alternative line (not shown).
However, proper operation of the FIG. 1 system depends upon, for example, proper parity error checking. But, since parity is checked, for example, only every 8 bits (a byte), multiple-bit errors in a byte normally cannot be detected. This causes an undercount in bit errors and prevents detection of deterioration in transmission quality. There is another problem with the parity error checking system shown in FIG. 1. The addition of a redundant parity bit to, for example, every 7 data bits reduces transmission efficiency to a great extent.